1. Field of the Invention
The present invention relates to an inverter unit for supplying power to a load for which a wide range of current control is required, and especially to an inverter unit preferably applied to a lighting device for lighting a cold fluorescent light (hereinafter referred as CFL) capable of dimming, known as a high frequency lighting.
2. Description of the Prior Art
The inverter unit is a device for converting DC power into AC power, and has been used, as so called a reverse converter, for various electric devices. FIG. 8 is a circuit showing a conventional inverter unit used for an electric discharge tube. In FIG. 8, T51 is a step-up transformer for Royer oscillation circuit provided with a primary coil Np51, a secondary coil Ns51, and a feedback coil Nf51. Q51 and Q52 are NPN transistors for a switching operation, both incorporated together with the step-up transformer T51 into the Royer oscillation circuit. C51 is a capacitor for voltage resonance, and L51 is a choke coil. In this configuration, when the transistors Q51, Q52 are both in suitch off state, the voltage between the collector and emitter has a shape of sine-wave, and the voltage waves for both primary coil Np51 and secondary coil Ns51 of transformer T51 are also shaped in sine-wave. The choke coil L51 is connected to DC-DC converter described later, and CFL1 is connected to the output of step-up transformer T51. A high voltage in sine-wave is output at a frequency of tens of KHz units by the self-excited oscillation of this inverter unit, and then the cold fluorescent light CFL1 lights on. IC51 is an integrated circuit(IC) for controlling the base circuit of PNP transistor Q53 for the switching operation forming DC-DC converter, and operates as a step-down type chopper circuit.
This IC comprises an oscillator OSC for generating a triangular wave, two operational amplifiers A51, A52 for comparison, a PWM comparator COMP for comparing the output voltage of oscillator OSC with of either one of operational amplifiers A51 or A52, and an output transistor Q54 which is driven by this PWM comparator and drives said base of PNP transistor Q53 for the switching operation. In this IC, there are two operational amplifiers A51, A52 connected to another input circuit of PWM comparator for comparing with the oscillator OSC as described above, though, a higher voltage among the output voltages of these two operational amplifiers is compared with the output of oscillator OSC. The IC having the configuration described above is herein defined as an IC for controlling DC-DC converter, and it will be also referred as the IC for controlling DC-DC converter unless the internal configuration changes, even if it was used for other applications. D51 is a flywheel diode, and L52 is a choke coil. C52 is a capacitor, and the choke coil L52 and capacitor C52 form LC filter. C53 and R51 are a capacitor and a resistor, respectively, for determining the oscillation frequency, and R52, C54, R53 and C55 are a C-R element for the phase correction of operational amplifiers A51, A52 in IC51 for controlling DC-DC converter. Diodes D52 and D53 are provided to rectify the positive element of discharge current flowing into CLF1. R54 and C56 are a resistor and a capacitor forming a low-pass filter for shaping the current wave into a direct current. This filter output is connected to the (+)input terminal of operational amplifier A52 in IC51.
Namely, voltage proportional to the average for the positive cycle of discharge current is obtained, and this voltage is compared in the operational amplifier A52 with the internal reference voltage of IC51, and an output voltage proportional to the difference between both voltages is obtained. As shown in FIG. 9, this output voltage and the triangular wave output of oscillator OSC in IC51 are compared by PWM comparator. Namely, when the discharge current increases by any cause, the output voltage of operational amplifier A52 which is in error amplification, shifts from B line to A line. Consequently, the output of PWM comparator changes from C line to D line. Namely, a period of time in switch on state for the switching PNP transistor Q53 which is an output transistor, is shorter, and the output voltage of DC-DC converter decreases, and then the supply voltage of Royer oscillation circuit drops, so the discharge current decreases. Therefore, constant-current control for the discharge current is achieved. R55 and R56 are resistors for constant-regulating the output voltage of DC-DC converter, and these are one for detecting DC-DC converter output voltage, in order to constant-regulate the voltage of secondary coil Ns51 of step-up transformer T51, when CFL1 is not connected, or before the discharge starts. A connection point of the resistors R55, R56 is connected to the (+) input terminal of operational amplifier A51 in IC51 to form a negative feedback loop, so as to constant-regulate the output voltage of DC-DC converter. The outputs of operational amplifiers A51, A52 are OR-connected, such that higher one among the output voltages of operational amplifiers A51, A52, is preferentially input into PWM comparator.
It is well known that such a conventional inverter unit described above has a limitation in power conversion efficiency. Because, the total efficiency .eta. of inverter unit is given as: EQU .eta.=(efficiency of converter portion) * (efficiency of inverter portion)
In order to improve the total efficiency .eta., it is required to improve each efficiency. For instance, the major causes of the decrease in efficiency for the DC-DC converter are the switching losses by the switching transistor Q53 and the diode D51, and the copper loss by the choke coil L52. Therefore, these losses can not be completely eliminated. The conventional inverter unit has a number of parts, so it is very difficult to reduce the inverter unit in size and price.
The present invention is proposed to overcome the conventional problems described above, and has an object to provide an inverter allowing reduction in the number of parts used for an inverter unit which can improves the efficiency and allows current control in a wide range.